[clang-tools-extra] [llvm] [clang-tools-extra][docs] Use Furo theme to match clang (PR #222196)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor commented:

<img width="559" height="803" alt="Image" src="https://github.com/user-attachments/assets/76fde471-63d9-4d54-afc1-fab995567c6c" />

Drop-down lists are not rendered on some of the topics like "Clang-Change-Namespace", "Clang-Reorder-Fields", etc..

On previous version we could see them:

<img width="1033" height="813" alt="Image" src="https://github.com/user-attachments/assets/4443049c-4adb-42bc-b277-444b30b58a6e" />

Maybe it's by design, but why then it shows arrow that I could click and nothing happens.

https://github.com/llvm/llvm-project/pull/222196